How to Detect Ammonia Ice Deposits on Europa's Surface with SUDA on Europa Clipper.

Impact mass spectrometers such as the Cosmic Dust Analyser (CDA) on board the Cassini spacecraft have proven invaluable in determining the composition of the surfaces from which detected particles originate. However, connecting impact mass spectra with the composition of the striking particle is not straightforward and requires laboratory impact spectra of particles of known composition and speed. Such data has so far been acquired for minerals, some organic materials and water ice. In this study, we present cation and anion impact mass spectra of aluminum and iron particles striking ammonia ice, a possible surface component of outer solar system bodies such as Europa, Ariel, and Pluto. To reduce ambiguity in the mass lines appearing in the spectra, we performed impact experiments with ¹⁴NH₃ and ¹⁵NH₃ ice.

The resulting impact mass spectra demonstrate only a slight dependence on impact speed. The cation impact spectra are dominated by protonated ammonia cluster ions, (NH₃)NH₄, the abundance of which decreases monotonically with cluster size. Consistent with data obtained from gas-phase mass spectrometry experiments involving ammonia clusters, (NH₃)₄NH₄ clusters are overabundant in impact mass spectra at all impact speeds, suggesting that these clusters possess a particularly stable geometric configuration. This finding implies that cluster formation in hypervelocity experiments is well described by gas-phase chemistry. Unlike water ice, well-formed ammonia cluster impact mass spectra can be obtained at impact speeds as low as 1 km/s. This suggests that ammonia deposits could be detected even by instruments in low orbits around icy moons such as Ganymede.

We performed Monte Carlo simulations to verify whether the potential ammonia deposits on Europa, as identified in Galileo IR data by Emran (2026), can be detected by the Surface Dust Analyzer (SUDA) impact mass spectrometer on board the Europa Clipper spacecraft [6]. This spacecraft will conduct 49 low altitude Europa flybys starting in 2030. Our simulations clearly demonstrate that, if present, such deposits will be unambiguously identified.